1. Field of the Invention:
The present invention relates to an ink jet printer and more particularly to an ink jet printer capable of highly efficiently removing such factors preventing a normal injection and flying of ink droplets as air bubbles or solid matter generated in or having entered into a nozzle or a pressure chamber.
2. Description of the Prior Art:
Several systems have been hitherto devised and put into practice for the printing head of the ink jet printer. For instance, FIG. 1 shows an example of a system called drop-on-demand system in which a nozzle 5 and a pressure chamber 3 are filled with the ink led from an ink storing chamber 7 through a duct 9. When an electric pulse is applied to a piezoelectric transducer 4 from a pulse generator 10, a flexible wall 2 together with the piezoelectric transducer 4 is deflected toward the pressure chamber 3 by means of a piezoelectric effect, so that the pressure chamber 3 suddenly decreases in volume. The sudden decrease in volume causes a liquid pressure to be produced in the pressure chamber 3, and the liquid pressure causes the ink in the pressure chamber 3 to be jetted out as an ink droplet 6 through the nozzle 5. The reduction portion of the ink in the pressure chamber 3 is compensated by ink 8 stored in the ink storing chamber 7 which flows into the pressure chamber 3 through the duct 9.
By the way, there are various kinds of factors which prevent the normal injection and flying of ink droplets. Among them are air bubbles and solid matter present in the nozzle 5 or the pressure chamber 3 which are frequently generated in normal use. In other words, if there are air bubbles in the nozzle 5 or the pressure chamber 3, all or a part of the pressure produced in the pressure chamber 3 is absorbed by the air bubbles, resulting in such abnormalities as incapability of injection of ink droplets, or fluctuation in the flying speed, impossibility of straight flying, and scattering of ink droplets being separated into a large number of smaller droplets as ink droplets are jetted out. Moreover, if there is solid matter in the nozzle 5, the normal injection of ink is prevented, and in the extreme case, the nozzle is clogged, so that it becomes completely impossible to jet out any ink droplets. Although the existence of solid matter in the pressure chamber 3 does not immediately result in an abnormality, this causes clogging sooner or later, bringing about such problems as mentioned above.
The air bubbles and solid matter causing such abnormalities are considered to be generated in the following cases: when an abnormal shock is applied to a printing head 1 during a recording operation or standby of a printer (not shown), so that an air bubble is undesirably drawn in from the nozzle 5; when a noise overlaps with the electric signal applied to the piezoelectric transducer 4 from the pulse generator 10 during a recording operation, thereby to disorder a normal vibration of meniscus of the ink in the nozzle 5, so that an air bubble is undesirably drawn in from the nozzle 5; when the air dissolved in the ink separates out; and when the ambient temperature changes while the printer is in an inoperative state and consequently the ink thermally expands or contracts, so that an air bubble is undesirably drawn in from the nozzle 5. Moreover, solid matter is also generated through drying and setting of the ink in the nozzle 5 when the printing head 1 is left in an inoperative state for a long period of time or the environmental moisture is abnormally low, and solid matter is also generated by the entry into the nozzle 5 of the dust floating in the air or the paper powder generated from the recording paper. In addition, there are also cases where solid matter is included in ink from the first.
In order to remove such air bubbles and solid matter preventing the normal injection and flying of ink from the nozzle 5 of the printing head 1, such a method has been conventionally employed as applying to the ink a washing liquid pressure higher than a constant pressure (the method of applying the washing liquid pressure is not shown) in order to form a forced ink flow in the pressure chamber 3 and the nozzle 5 (e.g., Japanese Patent Laid-Open No. 150030/1977). It is, however, not sufficiently effective in removing air bubbles or solid matter to only form a forced ink flow by thus simply applying a washing liquid pressure. In other words, when air bubbles or solid matter is attached to the wall inside the nozzle 5 or the pressure chamber 3 or when they are present in the vicinity of the wall, the ink flow rate in these places is not sufficient, so that it is often impossible to remove them. Especially, when the printing head 1 is not horizontally held, unlike the one shown in FIG. 1, but obliquely held so that the side of the nozzle 5 is lower than the other, air bubbles tend to move in the opposite direction to the nozzle 5 by means of the buoyancy thereof. In such a case, it is almost impossible to remove the air bubbles.
When air bubbles or solid matter cannot be removed from the pressure chamber 3 or the nozzle 5 as described above, there is hitherto such a problem that an expensive ink is uselessly made to flow out, since it is necessary to repeat the operation of forming an ink flow many times. When the printing head 1 is not restored to normal on the printer, the printing head is regarded as defective, and it is necessary to refill ink after the printing head 1 is removed from the printer. In the extreme case, the expensive printing head 1 is abandoned as defective.
Moreover, in order to remove such air bubbles and solid matter preventing the normal injection of ink into nozzle 5 and normal ejection of the ink from the nozzle 5 of the printing head 1, a washing liquid pressure higher than a constant pressure is applied to the ink thereby to form a forced ink flow in the pressure chamber 3 and the nozzle 5. Moreover, if such a method is employed as applying a pulse voltage as shown in FIG. 2(a) to, for example, the piezoelectric transducer secured to the printing head 1 for providing a mechanical vibration, the air bubbles and solid matter attached to the walls inside the pressure chamber 3 and the nozzle 5 are also supplied with the energy for separation and are separated from the walls as well as removed to the outside of the nozzle 5 together with the turbulent ink flow. The efficiency of removing the air bubbles and the solid matter is, however, not necessarily high in the case of only applying a washing liquid pressure in order to form a forced ink flow as well as applying a mechanical vibration to the inside of the pressure chamber 3. In other words, as shown in FIG. 2(b), although the mechanical vibration permits the ink in the pressure chamber 3 to be pressurized or reduced in pressure, if a mechanical vibration having the same magnitude as that in printing is applied, the air bubbles increase in both volume and number since the reduction in pressure is too large. Consequently, the injection of ink droplets becomes abnormal.
Such a phenomenon is generally called cavitation. Also when solid matter is present in the ink, the solid matter often has minute air bubbles attached thereto. Therefore, the air bubbles also increase and aggravate the trouble.
Moreover, in case of only thus applying a washing liquid pressure is applied in order to form a forced ink flow and a mechanical vibration is also applied to the inside of the pressure chamber 3, the air bubbles or solid particles are removed and at the same time a new air bubble is drawn into the pressure chamber 3 from the nozzle 5. As a result, the overall removal efficiency is not raised very high. The reason why a new air bubble is drawn in may be considered as follows.
FIG. 3(a) shows the ink meniscus in the nozzle 5 after an ink droplet is ejected out in a recording operation. A reference numeral 5a shows the meniscus in the most convex state, while a reference numeral 5b shows the same in the most concave state. The ink meniscus vibrates while reciprocating between the positions 5a and 5b in accordance with the change in pressure in the pressure chamber 3. The reason why the meniscus does not further enter into the pressure chamber 3 when reaching the position 5b is that the reduction in pressure in the pressure chamber 3 is small or that the surface tension of the ink is large. When the reduction in pressure in the pressure chamber 3 is large or the ink surface tension is small, the ink meniscus is constricted at a position 5c, as shown in FIG. 3(b), and an air bubble is finally formed.
By the way, when the ink is being discharged from the nozzle 5 by applying a washing liquid pressure thereto, a nozzle surface 11 is covered with a thin ink film, as shown in FIG. 3(c). If a mechanical vibration is applied to the inside of the pressure chamber 3 while the nozzle surface 11 is thus covered with the thin ink film, the constriction 5c of the ink meniscus is produced at a position very close to the tip of the nozzle, since the nozzle diameter is small, i.e., less than 100 microns, so that an air bubble is formed and drawn into the pressure chamber 3, as shown in FIG. 3(d).
Thus, the overall removal efficiency is hitherto low, since air bubbles or solid matter is removed from the nozzle 5 and at the same time, a new air bubble is formed inside the nozzle 5 and drawn into the pressure chamber 3.